The invention is directed to providing a method for providing generally uniform stitch lengths in a sewing or embroidery, as well as a device for implementing the method.
In sewing and embroidery machines, the article or material to be sewn is transported in each case after the execution of a sewing stitch by a material transport device. Such material transport devices are, for example, material feeders located underneath a throat plate or movable embroidery frame.
Material feeders can feature one or more bars lying horizontally, which are sawtooth shaped on the side facing the article to be sewn. Following the execution of each sewing stitch, i.e. after the sewing needle is no longer in contact with the article to be sewn, the material feeder performs one or more cyclical movements, whereby the article is transported one or more increments further in the direction of sewing. The material feeder is thereby raised so far that the bars protrude through slot shaped openings in the stitching plate and come into contact with the article to be sewn. The article to be sewn is pressed against the stitching plate and/or against the bars reaching through the throat plate by a presser foot. The material feeder then executes a pushing movement in the direction of sewing, whereby the article to be sewn is transported one increment in the direction of sewing. After this, the material feeder is lowered again, so that the bars no longer protrude above the throat plate and return to their original position. The individual partial movements can be merged into a continuous motion sequence. In most sewing machines, the direction of sewing can be reversed by reversing the described motion sequence, so that the new direction of sewing runs in the opposite direction of the original direction of sewing. There are also sewing machine models in which the material feeder, in addition to the direction of sewing, and in an analogous manner, can also execute transport movements that are perpendicular to the direction of sewing, so that the material or the article to be sewn can be moved in two dimensions or in a sewing plane predefined by the upper surface of the throat plate. Sewing machines of this type can be used for the embroidery of small patterns. Alternatively, an embroidery frame can also be used for the embroidery of patterns. Instead of material feeders, for example, an embroidery frame which can be driven by two stepper motors is used for moving the article within the sewing plane, whereby the material or the article is clamped into this embroidery frame.
Following the execution of a sewing stitch, the embroidery frame is moved via both stepper motors in such a way that the new stitching site is positioned underneath the sewing needle. For certain sewing procedures, and especially for the embroidery of patterns, it is of great importance that predetermined stitch lengths and directions within the sewing plane be observed. In conventional sewing and embroidery machines, the actual stitch lengths and directions can deviate, however, from the values set on the machine or calculated by the machine's control system. The actual material feeding in one or two directions during the individual transport steps or cycles does not correspond to the required specified values. Such deviations may be either system-contingent or random. Deviations of the actual stitch lengths or feeding increments from the respective target stitch lengths or target feeding increments of the material transport device may depend, for example, on the sewing machine model, or on the characteristics of the article or the material, or on the force effects on the article to be sewn when sewing or embroidering. Of particular importance is the sewing material-dependent slippage during the transport procedure or different transport characteristics of forwards and backwards transport of the material. Deviations of the actual values from the target values can also occur when using embroidery frames, for example, when the material buckles within the embroidery frame.
With deviations in the actual stitch lengths and/or the actual feeding increments from the target stitch lengths and/or target feeding increments, incorrect seam lengths or undesired misalignment of embroidery patterns can occur. It is not possible for conventional sewing machines to return the article to its original position by forwards and subsequent backwards transport with an equal number of each of a certain number of transport cycles. The same also applies to two-dimensional movement in the sewing plane. Incorrect seam lengths or cumulative misalignments of embroidery patterns can be the result.
A sewing machine with a device for measuring and regulating the size of the feeding increment is known from DE-C2-3525028. In the third embodiment, two CCD sensors situated opposite each other and vertically to the direction of sewing, with each being a line scan camera equipped with a light source. The line scan camera located to the front of the direction of sewing is switched on at the start of the sewing procedure and generates a digitalized real time line scan of a segment of the surface of the article. As soon as this segment of the surface is supposed to lie over the line scan camera situated to the rear in the direction of sewing according to the feeding speed, this line scan camera is switched on and scans the surface of the article until the pattern correlates with the pattern recorded beforehand by the forward line scan camera. A disadvantage of this device consists of its sensitivity to displacements which are perpendicular to the direction of sewing and to distortions of the article being sewn within the sewing plane. Even the smallest alterations in the position of the article to be sewn can lead to large differences in the calculation of correlation values. Furthermore, the brightness of the light source must be adjusted to the background brightness of the material. Also, the material to be sewn must at least be pushed forward the amount of the distance between both of the line sensors, until a value for the deviation of the actual feeding speed of the material from the target feeding speed can be determined. The measuring and regulation device can comprehend such deviations only in the direction of the feeding. In addition, the actual feeding speed must be slower than the target feeding speed. Both the calculation of the feeding speed and the position of the article to be sewn are afflicted with measurement errors.